Transcript Q - Effingham County Schools

Thermal Energy
Temperature and Thermal Energy

Thermal Energy

The study of heat transformations into other forms of energy is called
thermodynamics

Although the study of thermodynamics began in the eighteenth
century, it was not until around 1900 that the concepts of
thermodynamics were linked to the motions of atoms and molecules in
solids, liquids, and gases

We have already studied how objects collide and trade kinetic energies

The molecules may also have potential energy in their vibrations and
bending. The gas molecules collide with each other and with the walls
of their container, transferring energy among each other in the process
Temperature and Thermal Energy

Thermal Energy

There are numerous molecules moving freely in a gas, resulting in
many collisions

Therefore, it is convenient to discuss the total energy of the molecules
and the average energy per molecule

The total energy of the molecules is called thermal energy, and the
average energy per molecule is related to the temperature of the gas
Temperature and Thermal Energy

Hot Objects

If you put a balloon in sunlight, the balloon gets slightly larger

The energy from the Sun makes each of the gas atoms move faster and
bounce off the rubber walls of the balloon more often
Temperature and Thermal Energy

Hot Objects

Each atomic collision with the balloon wall puts a greater force on the
balloon and stretches the rubber. Thus, the balloon expands
Temperature and Thermal Energy

Hot Objects

On the other hand, if you refrigerate a balloon, you will find that it
shrinks slightly

Lowering the temperature slows the movement of the helium atoms

Hence, their collisions do not transfer enough momentum to stretch the
balloon

Even though the balloon contains the same number of atoms, the
balloon shrinks
Temperature and Thermal Energy

Solids

The atoms in solids also have kinetic energy, but they are unable to
move freely as gas atoms do

One way to illustrate the molecular structure of a solid is to picture a
number of atoms that are connected to each other by springs. Because
of the springs, the atoms bounce back and forth, with some bouncing
more than others

Each atom has some kinetic energy and some potential energy from
the springs that are attached to it

If a solid has N number of atoms, then the total thermal energy in the
solid is equal to the average kinetic energy and potential energy per
atom times N
Temperature and Thermal Energy

Thermal Energy and Temperature

A hot object has more thermal energy than a similar cold object

This means that, as a whole, the particles in a hot object have greater
thermal energy than the particles in a cold object

This does not mean that all the particles in an object have exactly the
same amount of energy; they have a wide range of energies

However, the average energy of the particles in a hot object is higher
than the average energy of the particles in a cold object
Temperature and Thermal Energy

Temperature

Temperature is a property of atoms themselves, and therefore, it does
not depend on the number of atoms in an object

Temperature depends only on the average kinetic energy of the
particles in the object
Temperature and Thermal Energy

Temperature

Consider two blocks of steel. The first block has a mass of 1 kg, and
the second block has a mass of 2 kg

If the 1 kg block is at the same temperature as the 2 kg block, the
average kinetic energy of the particles in each block is the same

However, the 2 kg block has twice the mass of the 1 kg block. Hence,
the 2 kg block has twice the amount of particles as the 1 kg block

Thus, the total amount of kinetic energy of the particles in the 2 kg
block is twice that of the 1 kg block
Temperature and Thermal Energy

Temperature

Average kinetic energy equals total kinetic energy divided by the total
number of particles in an object

Therefore, the thermal energy in an object is proportional to the
number of particles in it
Temperature and Thermal Energy

Equilibrium and Thermometry

How do you measure your body temperature?

If you suspect that you have a fever, you might place a thermometer in
your mouth and wait for a few minutes before checking the
thermometer for your temperature reading

The atomic level process involved in measuring temperature involves
collisions and energy transfers between the thermometer and your
body
Temperature and Thermal Energy

Equilibrium and Thermometry

When the cold glass tube of the thermometer touches your skin, which
is warmer than the glass, the faster-moving particles in your skin
collide with the slower-moving particles in the glass

Energy is then transferred from your skin to the glass particles by the
process of conduction, which is the transfer of kinetic energy when
particles collide

The thermal energy of the particles that make up the thermometer
increases, while at the same time, the thermal energy of the particles in
your skin decreases
Temperature and Thermal Energy

Thermal Equilibrium

As the particles in the glass gain more energy, they begin to give some
of their energy back to the particles in your body

At some point, the rate of transfer of energy between the glass and
your body becomes equal, and your body and the thermometer are
then at the same temperature
Temperature and Thermal Energy

Thermal Equilibrium

At this point, your body and the thermometer are said to have reached
thermal equilibrium, the state in which the rate of energy flow between
two objects is equal and the objects are at the same temperature, as
shown
Temperature and Thermal Energy

Thermal Equilibrium

The operation of a thermometer depends on some property, such as
volume, which changes with temperature

Many household thermometers contain colored alcohol that expands
when heated and rises in a narrow tube

The hotter the thermometer, the more the alcohol expands and the
higher it rises in the tube

Medical thermometers and the thermometers that monitor automobile
engines use very small, temperature-sensitive electronic circuits to
take rapid measurements
Temperature and Thermal Energy

Thermal Equilibrium

In liquid-crystal thermometers, such as the one shown in the figure, a
set of different kinds of liquid crystals is used

Each crystal’s molecules rearrange at a specific temperature, which
causes the color of the crystal to change and indicates the temperature
by color
Temperature and Thermal Energy

Temperature Scales: Celsius and Kelvin

Over the years, scientists developed temperature scales so that they
could compare their measurements with those of other scientists

A scale based on the properties of water was devised in 1741 by
Swedish astronomer and physicist Anders Celsius

On this scale, now called the Celsius scale, the freezing point of pure
water is defined to be 0°C

The boiling point of pure water at sea level is defined to be 100°C
Temperature and Thermal Energy

Temperature Limits

Objects with a wide variety of temperatures are present in the universe

Temperatures do not appear to have an upper limit. The interior of the
Sun is at least 1.5×107°C. Temperatures do, however, have a lower
limit
Temperature and Thermal Energy

Temperature Limits

If an ideal gas, such as helium in a balloon is cooled, it contracts in
such a way that it occupies a volume that is only the size of the helium
atoms at –273.15°C

At this temperature, all the thermal energy that can be removed has
been removed from the gas

It is impossible to reduce the temperature any further

Therefore, there can be no temperature lower than –273.15°C, which
is called absolute zero
Temperature and Thermal Energy

Temperature Limits

The Celsius scale is useful for day-to-day measurements of
temperature. It is not conducive for working on science and
engineering problems, however, because it has negative temperatures

Negative temperatures suggest a molecule could have negative kinetic
energy, which is not possible because kinetic energy is always positive

The solution to this issue is to use a temperature scale based on
absolute zero

The zero point of the Kelvin scale is defined to be absolute zero
Temperature and Thermal Energy

Temperature Limits

On the Kelvin scale, the freezing point of water (0°C) is about 273 K
and the boiling point of water is about 373 K

Each interval on this scale, called a kelvin,
is equal to 1°C

Thus, TC + 273 = TK
Temperature and Thermal Energy

Heat and the Flow of Thermal Energy

When two objects come in contact with each other, they transfer
energy

The energy that is transferred between objects when they come in
contact is called heat

The symbol Q is used to represent an amount of heat, which like other
forms of energy is measured in joules

If Q has a negative value, heat has left the object; if Q has a positive
value, heat has been absorbed by the object
Temperature and Thermal Energy

Conduction

If you place one end of a metal rod in a flame, the hot gas particles in
the flame conduct heat to the rod

The other end of the rod also becomes warm within a short period of
time

Heat is conducted because the particles in the rod are in direct contact
with each other
Temperature and Thermal Energy

Convection

Thermal energy transfer can occur even if the particles in an object are
not in direct contact with each other

Have you ever looked into a pot of water just about to boil?

The water at the bottom of the pot is heated by conduction and rises to
the top, while the colder water at the top sinks to the bottom

Heat flows between the rising hot water and the descending cold water

This motion of fluid in a liquid or gas caused by temperature
differences is called convection

Thunderstorms are excellent examples of large-scale atmospheric
convection
Temperature and Thermal Energy

Radiation

The third method of thermal transfer does not depend on the presence
of matter

The Sun warms Earth from more than 150 million km away via
radiation, which is the transfer of energy by electromagnetic waves

These waves carry the energy from the hot Sun to the much-cooler
Earth
Temperature and Thermal Energy

Specific Heat

Some objects are easier to heat than others

When heat flows into an object, its thermal energy and temperature
increase

The amount of the increase in temperature depends on the size of the
object, and on the material from which the object is made

The specific heat of a material is the amount of energy that must be
added to the material to raise the temperature of a unit mass by one
temperature unit

In SI units, specific heat, represented by C, is measured in J/kg·K
Temperature and Thermal Energy

Specific Heat

Liquid water has a high specific heat compared to the specific heat of
other substances

A mass of 1 kg of water requires 4180 J of energy to increase its
temperature by 1 K. The same mass of copper requires only 385 J to
increase its temperature by 1 K
Temperature and Thermal Energy

Specific Heat

The heat gained or lost by an object as its temperature changes
depends on the mass, the change in temperature, and the specific heat
of the substance

By using the following equation, you can calculate the amount of heat,
Q, that must be transferred to change the temperature of an object
Heat Transfer

Q = mCΔT = mC (Tf – Ti)
Heat transfer is equal to the mass of an object times the specific heat of
the object times the difference between the final and initial
temperatures
Temperature and Thermal Energy

Calorimetry: Measuring Specific Heat
Temperature and Thermal Energy

Calorimetry: Measuring Specific Heat

In an isolated, closed system, the change in thermal energy is equal to
the heat transferred because no work is done

Therefore, the change in energy for each block can be expressed by the
following equation:
ΔE = Q = mCΔT
Temperature and Thermal Energy

Calorimetry: Measuring Specific Heat

The increase in thermal energy of block A is equal to the decrease in
thermal energy of block B. Thus, the following relationship is true:
mACAΔTA + mBCBΔTB = 0
Temperature and Thermal Energy

Calorimetry: Measuring Specific Heat

The change in temperature is the difference between the final and
initial temperatures; that is, ΔT = Tf – Ti

If the temperature of a block increases, Tf >Ti, and ΔT is positive. If the
temperature of the block decreases, Tf <Ti, and ΔT is negative

The final temperatures of the two blocks are equal. The following is
the equation for the transfer of energy:
mACA(Tf – TA) + mBCB(Tf – TB) = 0
Temperature and Thermal Energy

Transferring Heat

Some of the best pots and pans for cooking are made of copper. To
learn why, compare copper pots to aluminum pots. Assume that 5.00 x
104 J of heat is added to a copper pot and an aluminum pot. Each pot
has a mass of 2.2 kg.

How much does the temperature of the copper pot increase (specific
heat = 385 J/kgK)?

How much does the temperature of the aluminum pot increase
(specific heat = 897 J/kgK)?
Temperature and Thermal Energy

Transferring Heat

There is 32.7 L of water in a bathtub. The temperature of the water is
42.1°C. If 11.3 L of water at 27.0°C is added to the bathtub, what is
the new temperature of the bathwater?
Laws of Thermodynamics

Changes of State

The figure diagrams the changes of state as thermal energy is added to
1.0 g of water starting at 243 K (ice) and continuing until it reaches
473 K (steam)
Laws of Thermodynamics

Changes of State

At some point, the added thermal energy causes the particles of a solid
to move rapidly enough that their motion overcomes the forces holding
them together in a fixed location

The particles are still touching each other, but they have more freedom
of movement. Eventually, the particles become free enough to slide
past each other
Laws of Thermodynamics

Melting Point

At this point, the substance has changed from a solid to a liquid

The temperature at which this change occurs is the melting point of the
substance

When a substance is melting, all of the added thermal energy goes to
overcome the forces holding the particles together in the solid state

None of the added thermal energy increases the kinetic energy of the
particles
Laws of Thermodynamics

Melting Point

This can be observed between points B and C in the figure, where the
added thermal energy melts the ice at a constant 273 K

Because the kinetic energy of the particles does not increase, the
temperature does not increase between points B and C
Laws of Thermodynamics

Boiling Point

Once a solid is completely melted, there are no more forces holding
the particles in the solid state

Adding more thermal energy again increases the motion of the
particles, and the temperature of the liquid rises
Laws of Thermodynamics

Boiling Point

In the diagram, this process occurs between points C and D
Laws of Thermodynamics

Boiling Point

As the temperature increases further, some particles in the liquid
acquire enough energy to break free from the other particles

At a specific temperature, known as the boiling point, further addition
of energy causes the substance to undergo another change of state

All the added thermal energy converts the substance from the liquid
state to the gaseous state
Laws of Thermodynamics

Boiling Point

As in melting, the temperature does not rise while a liquid boils

In the figure, this transition is represented between points D and E
Laws of Thermodynamics

Boiling Point

After the material is entirely converted to gas, any added thermal
energy again increases the motion of the particles, and the temperature
rises

Above point E, steam is heated to temperatures greater than 373 K
Laws of Thermodynamics

Heat of Fusion

The amount of energy needed to melt 1 kg of a substance is called the
heat of fusion of that substance

The added energy causes a change in state but not in temperature
Laws of Thermodynamics

Heat of Fusion

The horizontal distance in the figure from point B to point C represents
the heat of fusion
Laws of Thermodynamics

Heat of Fusion

The heat, Q, required to melt a solid of mass m is given by the
following equation
Heat Required to Melt a Solid

Q = mHf
The heat required to melt a solid is equal to the mass of the solid times
the heat of fusion of the solid
Laws of Thermodynamics

Heat of Vaporization

At normal atmospheric pressure, water boils at 373 K

The thermal energy needed to vaporize 1 kg of a liquid is called the
heat of vaporization. For water, the heat of vaporization is 2.26x106
J/kg
Laws of Thermodynamics

Heat of Vaporization

The distance from point D to point E in the figure represents the heat
of vaporization
Laws of Thermodynamics

Heat of Vaporization

The heat, Q, required to vaporize a mass, m, of liquid is given by the
following equation
Heat Required to Vaporize a Liquid

Q = mHv
The heat required to vaporize a liquid is equal to the mass of the liquid
times the heat of vaporization of the liquid
Laws of Thermodynamics

Heat of Vaporization

When a liquid freezes, an amount of heat, Q = –mHf, must be removed
from the liquid to turn it into a solid

The negative sign indicates that the heat is transferred from the sample
to the external world

In the same way, when a vapor condenses to a liquid, an amount of
heat, Q = –mHv, must be removed from the vapor
Laws of Thermodynamics

Heat of Vaporization

The values of some heats of fusion, Hf, and heats of vaporization, Hv,
are shown in the table below
Laws of Thermodynamics

The First Law of Thermodynamics

The first law of thermodynamics states that the change in thermal
energy, ΔU, of an object is equal to the heat, Q, that is added to the
object minus the work, W, done by the object
The First Law of Thermodynamics
ΔU = Q – W
Laws of Thermodynamics

The First Law of Thermodynamics

Thermodynamics also involves the study of the changes in thermal
properties of matter

The first law of thermodynamics is merely a restatement of the law of
conservation of energy, which states that energy is neither created nor
destroyed, but can be changed into other forms
Laws of Thermodynamics

Heat Engines

The warmth that you experience when you rub your hands together is
a result of the conversion of mechanical energy into thermal energy

However, the reverse process, the conversion of thermal energy into
mechanical energy, is more difficult

A device that is able to convert thermal energy to mechanical energy,
continuously, is called a heat engine
Laws of Thermodynamics

Heat Engines
Laws of Thermodynamics

Heat Engines

When the automobile engine is functioning, the exhaust gases and the
engine parts become hot

As the exhaust comes in contact with outside air and transfers heat to
it, the temperature of the outside air is raised

In addition, heat from the engine is transferred to a radiator

Outside air passes through the radiator and the air temperature is raised
Laws of Thermodynamics

Heat Engines

All of this energy, QL, transferred out of the automobile engine is
called waste heat, that is, heat that has not been converted into work

When the engine is working continuously, the internal energy of the
engine does not change, or ΔU = 0 = Q – W

The net heat going into the engine is Q = QH – QL. Thus, the work
done by the engine is W = QH – QL.
Laws of Thermodynamics

Efficiency

Engineers and car salespeople often talk about the fuel efficiency of
automobile engines

They are referring to the amount of the input heat, QH, that is turned
into useful work, W

The actual efficiency of an engine is given by the ratio W/QH. The
efficiency could equal 100 percent only if all of the input heat were
turned into work by the engine

Because there is always waste heat, even the most efficient engines fall
short of 100-percent efficiency
Laws of Thermodynamics

Refrigerators

Heat flows spontaneously from a warm object to a cold object

However, it is possible to remove thermal energy from a colder object
and add it to a warmer object if work is done

A refrigerator is a common example of a device that accomplishes this
transfer with the use of mechanical work

Electric energy runs a motor that does work on a gas and compresses it
Laws of Thermodynamics

Refrigerators

The gas draws heat from the interior of the refrigerator, passes from
the compressor through the condenser coils on the outside of the
refrigerator, and cools into a liquid

Thermal energy is transferred into the air in the room

The liquid reenters the interior, vaporizes, and absorbs thermal energy
from its surroundings
Laws of Thermodynamics

Refrigerators

The gas returns to the compressor and the process is repeated

The overall change in the thermal energy of the gas is zero
Laws of Thermodynamics

Refrigerators

Thus, according to the first law of thermodynamics, the sum of the
heat removed from the refrigerator’s contents and the work done by
the motor is equal to the heat expelled, as shown in the figure
Laws of Thermodynamics

Heat Pumps

A heat pump is a refrigerator that can be run in two directions

In the summer, the pump removes heat from a house and thus cools the
house

In the winter, heat is removed from the cold outside air and transferred
into the warmer house

In both cases, mechanical energy is required to transfer heat from a
cold object to a warmer one
Laws of Thermodynamics

The Second Law of Thermodynamics

Many processes that are consistent with the first law of
thermodynamics have never been observed to occur spontaneously

Two such processes are presented below
Laws of Thermodynamics

The Second Law of Thermodynamics

If heat engines completely converted thermal energy into mechanical
energy with no waste heat, then the first law of thermodynamics would
be obeyed

However, waste heat is always generated, and randomly distributed
particles of a gas are not observed to spontaneously arrange
themselves in specific ordered patterns
Laws of Thermodynamics

The Second Law of Thermodynamics

In the nineteenth century, French engineer Sadi Carnot studied the
ability of engines to convert thermal energy into mechanical energy

He developed a logical proof that even an ideal engine would generate
some waste heat

Carnot’s result is best described in terms of a quantity called entropy,
which is a measure of the disorder in a system
Laws of Thermodynamics

The Second Law of Thermodynamics

Entropy, like thermal energy, is contained in an object

If heat is added to an object, entropy is increased. If heat is removed
from an object, entropy is decreased

If an object does work with no change in temperature, the entropy does
not change, as long as friction is ignored
Laws of Thermodynamics

The Second Law of Thermodynamics

The change in entropy, ΔS, is expressed by the following equation, in
which entropy has units of J/K and the temperature is measured in
kelvins

The change in entropy of an object is equal to the heat added to the
object divided by the temperature of the object
Laws of Thermodynamics

The Second Law of Thermodynamics

The second law of thermodynamics states that natural processes go in
a direction that maintains or increases the total entropy of the universe
Laws of Thermodynamics

The Second Law of Thermodynamics

That is, all things will become more and more disordered unless some
action is taken to keep them ordered
Laws of Thermodynamics

The Second Law of Thermodynamics

The increase in entropy and the second law of thermodynamics can be
thought of as statements of the probability of events happening

The second law of thermodynamics predicts that heat flows
spontaneously only from a hot object to a cold object
Laws of Thermodynamics

Violations of the Second Law

We take for granted many daily events that occur spontaneously, or
naturally, in one direction

You are not surprised when a metal spoon, heated at one end, soon
becomes uniformly hot. Consider your reaction, however, if a spoon
lying on a table suddenly, on its own, became red hot at one end and
icy cold at the other

This imagined reverse process would violate the second law of
thermodynamics

It is just one example of the countless events that do not occur because
their processes would violate the second law of thermodynamics
Laws of Thermodynamics

Violations of the Second Law

The second law of thermodynamics and the increase in entropy also
give new meaning to what has been commonly called the energy crisis

The energy crisis refers to the continued use of limited resources of
fossil fuels, such as natural gas and petroleum

When you use a resource, such as natural gas to heat your home, you
do not use up the energy in the gas
Laws of Thermodynamics

Violations of the Second Law

As the gas ignites, the internal chemical energy contained in the
molecules of the gas is converted into the thermal energy of the flame

The thermal energy of the flame is then transferred to thermal energy
in the air of your home

Even if this warm air leaks to the outside, the energy is not lost.
Energy has not been used up. The entropy, however, has increased
Laws of Thermodynamics

Violations of the Second Law

The chemical structure of natural gas is very ordered

As you have learned, when a substance becomes warmer, the average
kinetic energy of the particles in the substance increases

In contrast, the random motion of warmed air is very disordered

While it is mathematically possible for the original chemical order to
be re-established, the probability of this occurring is essentially zero
Laws of Thermodynamics

Violations of the Second Law

For this reason, entropy often is used as a measure of the unavailability
of useful energy

The energy in the warmed air in a home is not as available to do
mechanical work or to transfer heat to other objects as the original gas
molecules were

The lack of usable energy is actually a surplus of entropy
Laws of Thermodynamics

Laws of Thermodynamics

A calorimeter containing 1.2 kg of water at 23.0°C is used to find the
specific heat of a number of substances. A 503 g sample of each
substance is heated to 250°C and is placed in the calorimeter. For each
of the following final temperatures, calculate the specific heat of the
substance is

31.5°C

32.8°C

41.8°C

28.2°C
Laws of Thermodynamics

Laws of Thermodynamics

How much heat is required to completely vaporize a 23.0 kg block of
ice at -8.00°C